1. Field of the Invention
The present invention relates to a torque measuring apparatus attached to an output shaft of, for example, an actuator motor and to measure a torque applied to the output shaft and an actuator drive control system, and particularly to a torque measuring apparatus in which strain gauges are bonded to a flexure element to generate strain by the application of torque and the torque is measured based on output signals of the strain gauges, and an actuator drive control system.
2. Description of the Related Art
A torque sensor is studied and developed from long ago. The torque sensor is used for a drive system of, for example, an actuator control apparatus of force control system, or “haptic device” for providing the sense of force or the sense of touch to the operator in addition to visual information and aural information. Here, the force control is a control method in which a target value of force to be applied to a working object is directly received, and the force represented by the target value is realized. The output torque is detected more accurately and feedback is performed, so that a more flexible interpersonal physical interaction service in force order becomes possible. The main application of the haptic device is to learn a specific technique in medicine or others, or to provide the three-dimensional sense of force and sense of touch on a material in an environment in which it cannot be actually touched, such as in remote control working in a virtual environment such as small cosmos or ocean, or in a specific or dangerous environment such as a nuclear reactor. The output torque is detected more accurately and the feedback is performed, so that the feeling obtained when the material in the virtual environment is touched or grasped can be realistically provided to the user through the haptic device.
FIG. 16 shows a general structural example for measuring the output torque of an actuator motor. In the illustrated example, a coupling is disposed between an output shaft of the actuator through a speed reducer and a load object (not shown), and a pair of strain gauges are bonded to the cylindrical surface of the coupling in such a way that each of them is deflected by 45 degrees with respect to the rotation axis direction (that is, they draw approximately a letter X). When the coupling is deformed by load torque applied to the output shaft, the electric resistance values of the strain gauges united with the coupling are also changed due to the deformation. Accordingly, the deformation amount of the coupling, that is, the applied torque can be obtained in accordance with the combination of the change amounts of inter-terminal voltages of the respective strain gauges. Besides, when the output shaft is rotated infinitely like a general rotation transmission system, it is sufficient if the torque is measured from the outside through a contact such as a slip ring.
A harmonic drive speed reducer typified by “Harmonic Drive (registered trademark)” of Harmonic Drive Systems Inc. includes a wave generator as an actuator input, a circular spline as an output shaft, and a flex spline at a fixed side (supporting counter torque). In the accurate power transmission system as stated above, when torque measurement is performed using strain gauges, as shown in FIG. 17, plural pairs of strain gauges are coaxially bonded to a member to fix the circular spline, and the torque can be measured based on the change of electric signals corresponding to the change amounts of the respective strain gauges. When attention is paid to the harmonic drive speed reducer, since the strain of the member constituting the flex spline is observed, the counter torque is measured. Since the strain gauge itself is a sensitive sensor, it can be said that the characteristic of the harmonic drive speed reducer determines the accuracy of the torque measuring method. Besides, the periodically undulated deformation is smoothened by using the plural pairs of strain gauges, and the effect of extracting only the deformation due to the torque around the output shaft can be expected.
As shown in FIG. 18, a float differential torque measuring apparatus has a structure in which an output shaft of a motor with a speed reducer is supported from the outside by bearings and can be freely rotated (see, for example, Cowritten by Hirose and Kato, “Development of Float Differential Torque Sensor” (Abstracts for meeting of the Japan Society of Mechanical Engineers, Robotics and Mechatronics, ICI2-6, 1998) (non-patent document 1)). The outside is supported by an elastic beam, and when torque is applied to the output shaft, counter torque is applied through the passage of output shaft→speed reducer→motor rotator→motor stator→casing→beam. The characteristic of the beam is adjusted according to the measuring method, and a strain gauge can be used. In the illustrated apparatus, the beam is twisted by the counter torque, and an auxiliary beam is used to absorb the displacement of the beam in the axial direction. At least two sets of beams are provided, the strain gauge is disposed on each of them, and wire connection is performed to cancel strain other than strain due to the torque.
Since the float differential torque measuring apparatus measures the counter torque even in the state where the backdrivability of the speed reducer is very low, there is a merit that the torque measurement is possible. However, when the motor is a measuring object, since the measurement of the torque is performed through the passage of load→speed reducer→rotator→gap→stator→beam, the repulsive force between the rotator and the stator is merely measured. For example, even if an impulse force is exerted on the load side, it is merely transmitted as the repulsive force to the sensor through the electromagnetic system, a signal of high frequency cannot be detected.
On the other hand, a torque measuring method can be mentioned, which uses a characteristic that a motor drive current and an output torque are almost uniquely determined. In recent years, an inexpensive and highly sensitive current measuring device such as a Hall device can be used. There is often adopted such a structure that the Hall device is disposed at the output stage of a motor drive circuit, and current control, that is, torque control is performed. However, since not only the characteristic of a load, but also the characteristics of a halfway transmission system, a speed reducer and the like are also included, it is difficult to perform accurate force (torque) control to the object. Besides, it is conceivable that a dead zone and nonlinearlity due to friction and hysteresis occurs, the sensor value includes these, and the characteristic of the sensor system is remarkably reduced. Although the nonlinearlity is not fatal, the dead zone causes a minute component to be unable to be detected, the reproducibility of the hysteresis is lost, and the accuracy is impaired.
Besides, as a general torque measuring method, a flexure element to generate strain by the application of torque is attached to an output shaft of an actuator motor, strain gauges are bonded to specified parts of the flexure element, and the torque is calculated based on electric signals outputted from the respective strain gauges.
For example, there is proposed a load sensor which includes a flexure element to generate strain by a load, and a detection element disposed on the flexure element and having a detection value changed according to the strain, and in which the strain of the flexure element deformed in the rotation axis direction of a brake disk is detected according to a brake torque to brake the rotation of the brake disk (see, for example, JP-A-2008-76161 (patent document 1)).
Besides, there is proposed a torque sensor which is directly provided on the outside surface of a cylindrical flexure element, and in which a stress due to a torsion torque generated in the flexure element is directly transmitted to a first compression strain resistance element, a second compression strain resistance element, a first tensile strain resistance element, and a second tensile strain resistance element (see, for example, JP-A-2007-155576 (patent document 2)).
Besides, there is proposed a torque detector in which a torque to be detected is applied to a flexure element of a strain gauge through an elastic body, and breakage of the flexure element, and peeling of a resistance body are prevented (see, for example, JP-A-9-138168 (patent document 3)).
Besides, there is proposed a low cost torque sensor in which a flexure element deformed by relative rotation displacement to move a slider in an axial line direction is used, and a dead zone is not generated (see, for example, JP-A-8-327472 (patent document 4)).
The strain gauge is a device whose electric resistance value is almost linearly changed according to the strain amount, and is known as a sensitive sensor (described before). Accordingly, the present inventors consider that a torque measuring apparatus with higher accuracy can be constructed when a flexure element to generate strain only in a direction in which a torque is applied (in other words, the generation of strain in a direction other than an expected direction can be suppressed) is attached to an output shaft of an actuator motor, and strain gauges are bonded to the surface of the flexure element as stated above. That is, since the torque is the moment of a force around a rotation shaft, the flexure element has a part mainly displaced around the rotation shaft according to the torque, and when the strain gauges are bonded to the part, the torque could be measured at high accuracy based on electric signals outputted from the strain gauges.